Mattress



FIG. 1 is a left side, top perspective view of a mattress showing my new design;

FIG. 2 is a left side view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a front view thereof; and,

FIG. 5 is a rear view thereof.

The broken lines are environmental and form no part of the claimed design. 

CLAIM The ornamental design for a mattress, as shown and described. 